Linked compressor and expander (Lincompex) systems are known in the telecommunications arts. These systems were originally implemented with analog technology but may now be implemented digitally such as taught in U.S. Pat. No. 4,271,499 to Leveque, the inventor of the present application. In the past, such Lincompex systems have been generally utilized only to transmit audible speech. While the Leveque '499 patent does disclose the possibility of transmitting data between speech syllables to optimize transmitter loading in such a system primarily intended for transmission of speech the system proposed for transmission of data in the '499 Leveque patent does not transmit the data through the Lincompex compressor and expander and thus does not derive the benefits of Lincompex for the data.
Known Lincompex systems such as that taught by the '499 Leveque patent may transmit voice using Lincompex techniques as the voice signal is band limited and exhibits an envelope which is also band limited and does not overlap the voice band. Referring to FIG. 2(a), a typical voice frequency band 2 is illustrated. Normal voice is band limited between 300 and 2700 hertz and has an envelope which exhibits an envelope signal frequency band 4 of considerably less than 300 hertz. Normally, the envelope for voice exhibits a frequency band from 0 to 90 Hz and the envelope signal, when modulated according to the teachings of the '499 patent, has a center frequency of 2900 Hz. While Lincompex systems such as that disclosed in the '499 Leveque patent excel in situations such as that illustrated in FIG. 2(a) where the envelope and voice bands do not overlap, the Lincompex systems utilized in the past as exemplified by the '499 Leveque patent were not heretofore useable to transmit data where the data band, when modulated, and the envelope signal band overlap.
The data signal band and its envelope band will often overlap where a high peak/average signal is utilized, particularly where random signal content is encountered. A good example of such a high peak/average signal is so-called "parallel-tone" or "multi-tone data" which is often utilized to transmit data across radio links or other communication channels. Such parallel-tone or multi-tone data utilizes a plurality of tones which may be modulated by one of several different types of data modulation such as amplitude shift keying (ASK), frequency shift keying (FSK), phase-shift keying (PSK), phase modulation (PM), frequency modulation (FM), or the like. Assuming a transmitter has a maximum power P.sub.max, and assuming a n-tone, parallel-tone or multi-tone system is utilized, then the average power per tone is shown by the equation: EQU P.sub.tone =P.sub.max /n.sup.2
Therefore, in a 16 tone system when P.sub.max equals 1000 watts, P.sub.tone equals 1000/256 or about 4 watts. The average power per tone is about 24 dB down from the peak power of the transmitter. This is a very inefficient system. In contrast, according to the teachings of the present application, Lincompex techniques may be used in such data transmission thereby dramatically increasing efficiency.
The problem associated with the transmission of such parallel-tone or multi-tone data using Lincompex techniques is best illustrated in FIGS. 2(b) and 3. FIG. 2(b) illustrates a situation where the band width of the data 10, the spectrum of tones of the parallel-tone or multi-tone data, overlaps the envelope signal generated by the Lincompex system. The compression process is accomplished by a circuit shown in FIG. 1(a). The circuit of FIG. 1(a) would accomplish the compression of the data signal represented in FIG. 2(b) by dividing the spectrum of the input signal by the overlapping spectrum of that signals envelope thereby creating undesirable singularities. These singularities make it impossible to recover the data. Thus, in the past, Lincompex techniques could not be used to compress data of the type exhibited in FIG. 2(b).
A similar problem is exhibited in FIG. 3. In FIG. 3, the data band width 10 has a frequency band between frequencies A and B. When this data is modulated, the modulated data spectrum 14 can increase to extend from A' to B'. Thus, while the data spectrum 10 and the envelope spectrum 12 do not, in and of themselves overlap, the modulated data spectrum overlaps the envelope spectrum again making data compression and recovery difficult when using Lincompex techniques.
FIG. 1 of the present application illustrates a Lincompex system similar to that illustrated in the '499 patent. In such a Lincompex system, voice information to be transmitted is introduced to an input 20 of the Lincompex system. A control tone generator or envelope circuit 24 monitors the input voice signal. An envelope detector 26 of the control tone generator (envelope circuit) 24 detects the envelope of the introduced voice signal and develops an envelope signal having a voltage representative of the signal level of the introduced voice signal. A compressor 22 compresses the introduced input voice signal. Compression is performed by dividing the signal by its envelope in pseudo-real time to produce a compressed voice signal. To develop the control tone, the system of FIG. 1 supplies the envelope signal developed from the output of the envelope detector 26 to a logarithmic (log) amplifier 28 which then develops a signal representative of the logarithm of the envelope signal. The output of this logarithmic amplifier 28 is supplied to a control terminal of a voltage control FM oscillator 30 which generates a frequency which varies about a center frequency F.sub.c in relation to the variation of the input voltage supplied to its control terminal from the logarithmic amplifier 28 to develop an envelope signal as an output of the control tone generator (envelope circuit) 24.
A summer 32 then sums the compressed voice signal developed at the output of the compressor 22 with the envelope signal developed at the output of the FM oscillator 30 to form a combined information signal.
In a speech transmission system, this summed waveform has a frequency band such as that illustrated in FIG. 2(a). A transmitter 34 is then input with the combined information signal produced by the summer 32 and transmits the signal over a desired transmission medium 36. In a typical embodiment, a single side band transmitter would normally transmit the modulated combined information signal across the airways in a known manner.
A conventional Lincompex demodulator is illustrated in FIG. 1(b). This demodulator receives the modulated combined information signal from the transmission medium 36a, which, once again, normally includes an antenna receiving radio waves from the atmosphere, and supplies the received modulated combined information signal to a receiver 38 which demodulates the transmitted signal to reproduce the combined information signal. Typically, this receiver would be a single side band receiver which mixes the received modulated combined information signal with the carrier frequency to produce a base band combined information signal. When transmitting voice, the combined information signal will exhibit the characteristics illustrated in FIG. 2(a).
To recover only the voice from such a combined information signal, a low pass filter 40 removes the envelope information 4 of FIG. 2(a) from the combined information signal to recover the compressed voice signal containing only the voice information 2. This compressed voice information is transmitted according to the Lincompex techniques at a substantially constant syllabic peak voltage which enables substantially complete modulation of the transmitter 34 of FIG. 1(a). This information must then be expanded to produce the necessary dynamic range for the recovered voice signal to be supplied at the output 52. Accordingly, an expander 42 is utilized which essentially multiplies the compressed voice signal developed at the output of low pass filter 40 by the envelope signal which is recovered by a control tone conversion circuit 44.
The control tone conversion circuit 44 comprises a band pass filter 46 which recovers only the envelope signal 4 of FIG. 2(a) from the combined information signal. This FM modulated envelope signal originally developed by the FM oscillator 30 of FIG. 1(a) is then frequency demodulated by a frequency demodulator 48 to recover the logarithm of the envelope. An anti-logarithm amplifier 50 is then utilized to recover the original envelope developed by the envelope detector 26 of FIG. 1(a). This original envelope signal is then used to recover the original voice signal by expanding the compressed voice signal via the expander 42 to provide the original signal to the output 52.
As mentioned above, the conventional Lincompex techniques are best performed digitally according to the teachings of the Leveque '499 patent. While these techniques are successful for transmitting voice which is band limited and which exhibits an envelope having a frequency band which does not overlap the frequency band of the voice signal, such a system cannot be applied to signals which have overlapping information and envelope spectra.
A typical form for encoding data for transmission across a communications channel such as a radio link is the well known parallel-tone or multi-tone data encodation technique. In a multi-tone data encodation technique, a plurality of fixed or variable tonal frequencies are utilized to construct the overall data waveform. In FIG. 3, the unmodulated multi-tone data 10 extends in the frequency range of A-B. While, with proper frequency constraints, the envelope signal 12 of FIG. 3 might not overlap the data 10, itself, when modulated by one of the aforementioned techniques, the modulated data 14 exhibits an increased bandwidth of A'-B', the data frequency range 14 therefore overlapping the envelope frequency range 12. In such a case where the envelope and data frequency spectra overlap, it is not possible to utilize the prior art system of the '499 Leveque patent to modulate data due to the overlap between the envelope signal and data signal.